The present invention relates to a phototransistor of the type having an emitter-base heterojunction for use in optical telecommunications.
Photodetectors used for optical telecommunications must have an excellent sensitivity, i.e. must be able to detect low amplitude optical signals. Avalanche photodiodes are frequently used for this purpose. They are then associated with low noise preamplifiers. However, problems associated with avalanche operation make this solution difficult to realize, particularly at wavelengths corresponding to the optimum transmission windows of optical fibres (1.3 and 1.55 .mu.m). Another solution consists of using a single PN or PIN photodiode associated with a preamplifier having a very low input capacitance. The input component of the preamplifier is either an e.g. GaAs field effect transistor or a bipolar transistor with a very high transition frequency (several GHz). These two solutions are compared in the article of SMITH, HOPPER and GARRET published in the journal "Optical and Quantum Electronics", 10, 1978, pp. 292-300.
The phototransistor which can be considered as the result of the monolithic integration of the photodiode and a transistor is also of some interest in photodetection because there is no input connection, provided that the photodiode and transistor elements have characteristics which are at least equivalent to those of the corresponding discrete components.
The present invention relates to a photodetector of the latter type. Besides phototransistors with a silicon homojunction, heterojunction phototransistors (of the Npn type) are known, which are essentially obtained from the materials composed of elements of columns III and V of the periodic classification of elements. They consist, for example, of GaAlAs-GaAs or InP-GaInAsP phototransistors.
All these phototransistors operate according to the same principle. The incident light is absorbed in the base layer, as well as in the free base-collector area, namely in the collector. The resulting photocurrent is superimposed on the base current of the phototransistor. There is a variation in the collector current which is equal to the primary photocurrent which multiplies the gain of the phototransistor.
With regards to heterojunction devices the phototransistor without a base contact for which the light traverses the emitter layer before being absorbed in the base layer has interesting characteristics, particularly with regards to sensitivity.
Such a phototransistor is shown in sectional form in FIG. 1. It comprises a layer 2 serving as a collector (e.g. of n-type GaAs), a layer 3 serving as the base (e.g. of p-type GaAs) and a layer 4 serving as the emitter (e.g. of N-type GaAlAs). All these layers are deposited on a substrate 12 (e.g. of N.sup.+ -type GaAs). The electrical contacts are provided by the emitter contact 7 and the collector contact 9.
This type of phototransistor is described in the articles of BENEKING et al. "Electronics Letters", vol. 12, 1976, pp. 395/6, MILANO et al. "IEEE International Electron Devices Meeting", December 1979 and finally KONAGAI et al. "Journal of Applied Physics", 48, 1977, pp. 4389-4394.
Other phototransistors have a base contact. In this case the emitter layer is etched in order to metallize the contact on the base. The light can then be directly focused on to the base layer in the etched area.
Such a phototransistor is shown in FIG. 2 with the same references as in FIG. 1. The base contact carries the reference numeral 8. This type of phototransistor was described by BENOIT at the Conference Internationale sur les Communications, Optiques, Paris 1976.
All the known devices have numerous disadvantages. The homojunction phototransistor is not very suitable for photodetection at the low wavelength levels envisaged in telecommunications by optical fibres (approx. 0.85 .mu.m). Thus, at this wavelength silicon has a relatively low absorption coefficient necessitating a considerable thickness of the free base-collector area (equal to or above 10 .mu.m) which is not very compatible with rapid transistor operation.
With regards to heterojunction phototransistors due note should be taken of the following points.
(a) In the case where the light traverses the emitter layer, as is the case for the phototransistor of FIG. 1, components are obtained whose emitter-base capacitance is necessarily high. For photodetection at the end of the optical fibre it is necessary to have a significant surface area with a diameter of approximately 50 .mu.m, which, even with the low level emitter doping used in heterojunction transistors (a few 10.sup.16 cm.sup.-3) leads to high capacitances (&gt;5 pF). Moreover, the recombination currents, whose amplitude is proportional to the surface of the emitter-base junction limit the possibility of operating at low current necessary in low noise photodetection. Conversely the sensitivity of these components in the transmission window of the emitter is excellent due to the low recombination speed of the carriers at the emitter-base interface. PA0 (b) If for the purpose of reducing the emitter-base capacitance and the recombination current the solution based on the locally etched emitter as for the phototransistor of FIG. 2 is adopted, there is the problem of the sensitivity reduction. Thus, the light which is absorbed in the base and collector underneath the etched portion, creates carriers whereof a by no means negligible part is recombined in the surface of the device. The free air-GaAs and also GaInAsP-air surfaces are in effect characterized by a high recombination speed (10.sup.5 cm/s for GaAs) and passivation tests of these surfaces have not as yet given satisfactory results. PA0 (c) Finally the selective etching of the emitter layer is not always easy, this operation increasing in difficulty as the base layer becomes finer, which requires a high current gain.